Grayscale mask technology is known to be one method for generating a range of photoresist thicknesses at a given exposure and development, using conventional photolithographic tools. The technique for generating the grayscale mask is not well established and the market for such product at this time is not particularly large. Yet, a low cost grayscale reticle may be used in many applications, from fabrication of MEMS structures to generating a dual damascene trench. It is especially useful in the field of microlens array formation, as used in an image sensor.
There are a number of ways to produce a microlens array for state of the art CCD and CMOS image sensor arrays. Currently, an array of circles is photolithography patterned into a layer of photoresist, generating cylindrically shaped resist blocks, which are then reflowed, or melted, at a high temperature to form hemispheric shapes. A dry etch then transfers the photoresist shape to a high-refractive-index material to form the microlens array. The shape of the feature is dictated by the flow process, which requires precisely defined temperature and duration, else, neighboring features will merge and flow together, destroying the desired lens shape. Thus, the distance between features needs to be relatively large. This method does not provide a particularly high-fill factor in the resulting microlens array.
A preferred alternative to the previously described technique is to use a gray scale photomask which controls the exposure over a wide range of values and generates a variety of resist thicknesses in the same layer. The proper design of the gray scale mask allows direct patterning of the microlens array into photoresist. Again, a dry etch transfers the pattern to the lens material. This enables an extremely high-fill factor on an array, approaching 100%. A significant drawback, however, is the high cost of generating a gray scale photomask, because fabrication of a grayscale mask is very difficult. The one proven, reliable method, known as high energy beam sensitive (HEBS) glass, requires several days of e-beam work, resulting in a very high cost fabrication.
U.S. Pat. No. 6,524,756, to Wu, granted Feb. 25, 2003, for Gray scale all-glass photomasks, describes a zinc silicate material that changes its light transmission properties as a function of the e-beam irradiation dose to generate a HEBS layer for the fabrication of a gray scale mask. The time required to generate a complicated array of grayscale features as in a microlens array is extremely long and the eventual cost is very high. The process uses a narrowly defined range of zinc silicate glass compositions is found to produce HEBS-glass which possesses the essential property of being a true gray level mask, which property is necessary for the fabrication of general three dimensional microstructures using a single optical exposure in a conventional photolithographic process.
U.S. Pat. No. 6,071,652, to Feldman et al., granted Jun. 6, 2000, for Fabricating optical elements using a photoresist formed from contact printing of a gray level mask, and U.S. Pat. No. 6,420,073 to Suleski et al., granted Jul. 16, 2002, for Fabricating optical elements using a photoresist formed from proximity printing of a gray level mask, describe processes which generates a structure similar to that shown in FIG. 1a, herein. Grayscale patterns are created by varying the thickness of a light absorbing layer. Multiple binary masks may be used to generate such thickness variations.
U.S. Pat. No. 6,033,766, to Block et al., granted Mar. 7, 2000, and U.S. Pat. No. 5,998,066 to Block et al., granted Dec. 7, 1999, both for Gray scale mask and depth pattern transfer technique using inorganic chalcogenide glass, describe use of an inorganic chalcogenide glass, such as a selenium germanium, coated with a thin layer of silver, a gray scale mask may be produced with accurate control of the size, uniformity and variance of the pixels. The selenium germanium glass is composed of column structures arranged perpendicularly to the substrate, resulting in a possible edge precision of 100 Å. The gray scale mask may be used to impress information as a modulated thickness on a selenium germanium photoresist layer on an inorganic substrate. The selenium germanium photoresist layer may then transfer the gray scale to the substrate.
U.S. Pat. No. 5,334,467 to Cronin et al., granted Aug. 2, 1994, for Gray level mask, and U.S. Pat. No. 5,213,916 to Cronin et al., granted May 25, 1993, for Method of making a gray level mask, describe use of a gray level mask suitable for photolithography, which is constructed of a transparent glass substrate which supports plural levels of materials having different optical transmissivities. In the case of a mask employing only two of these levels, one level may be constructed of a glass made partially transmissive by substitution of silver ions in place of metal ions of alkali metal silicates employed in the construction of the glass. The second layer may be made opaque by construction of the layer of a metal such as chromium. The mask is fabricated with the aid of a photoresist structure which is etched in specific regions by photolithographic masking to enable selective etching of exposed regions of the level of materials of differing optical transmissivities. Various etches are employed for selective etching of the photoresist, the metal of one of the layers, and the glass of the other of the layers. The etches include plasma etch with chloride ions to attack the chromium of the opaque layer, compounds of fluorine to attack the glass layer, and reactive ion etching with oxygen to attack the photoresist structure. Also, developer is employed for etching on hardened regions of resist in the photoresist structure.